1. Field of the Invention
The present invention relates to an electric double layer capacitor and, particularly, to constituents of a polarizable electrode of an electric double layer capacitor.
2. Description of the Prior Art
Recently, an electric double layer capacitor having high energy density and large power density has been requested as an energy source for driving a motor of such as electric car or an energy recovery system thereof.
In order to improve the power density in response to such request, a method for reducing an internal resistance of the electric double layer capacitor has been proposed. For example, Japanese Patent Application Laid-open No. H8-55761 discloses an electric double layer capacitor having polarizable electrodes constituted mainly with activated carbon having high specific surface and an auxiliary carbon material for giving electric conductivity to activated carbon, which are bound together by a binder such as fluorine containing polymer.
According to the disclosed electric double layer capacitor, the polarizable electrode is constituted with fluorine containing polymer selected from a group consisting of polyvinylidene fluoride, fluoroolefin vinyl ether copolymer and tetrafluoroethylene-propylene copolymer, activated carbon having large specific surface and acetylene black or ketjen black as the auxiliary carbon material.
On the other hand, Japanese Patent Application Laid-open No. H9-171946 discloses an electrode of an electric double layer capacitor using a material prepared by mixing epitaxially grown carbon fiber and resin and carbonating the mixture by heating it and a method for manufacturing the same electrode.
However, these prior arts have problems which are described below.
First, in the case of the electric double layer capacitor disclosed in Japanese Application Laid-open No. H8-55761, the reduction of internal resistance is not enough. The reason for this is that, since particle size of acetylene black or ketjen black is substantially smaller than that of activated carbon, only acetylene black or ketjen black agglomerate to form secondary particles, resulting in insufficient reduction of contact resistance between particles.
Second, electrostatic capacitance of the electric double layer capacitor disclosed in Japanese Patent Application Laid-open No. H9-171946 is small. The reason for this is that, since the carbonated material is used as the polarizable electrode without invigoration thereof, a surface area thereof, which can be utilized as the polarizable electrode, is small.
Third, even if the carbonated material of the polarizable electrode is invigorated in the electric double layer capacitor disclosed in Japanese Patent Application Laid-open No. H9-171946, the electrostatic capacitance and the internal resistance of the electric double layer capacitors manufactured by the described method are substantially varied. The reason for this is that, when the carbonated material is invigorated, the rigidity of the polarizable electrode is reduced and it becomes impossible to maintain a predetermined configuration of the electrode to thereby precisely put a necessary amount of polarizable electrode material in the electric double layer capacitor. Particularly, this phenomenon tends to be more actualized when the polarizable electrode is made thinner. It is necessarily possible to maintain the predetermined configuration of the electrode if the degree of invigoration is weak. However, since an enough surface area of the electrode can not given as mentioned previously, the electrostatic capacitance of the capacitor becomes small.